<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 400px;
            height: 200px;
            background-color: burlywood;
            /* border 撑大元素 */
            /* border: 30px solid rebeccapurple; */
            /* border-width: 3px; */
            /* 实线 */
            /* border-style: solid; */
            /* 虚线 */
            /* border-style: dashed; */
            /* 点线 */
            /* border-style: dotted; */

            /* border-color: red; */

            /* 上下左右 */
            border-top: 30px solid rebeccapurple;
            border-bottom: 20px solid red;
            border-left: 10px solid green;
            border-right: 1px solid aqua;

        }

        .triangle {
            margin-top: 50px;
            width: 0;
            height: 0;
            border-top: 10px solid red;
            /* transparent 透明色 */
            /* border-bottom: 30px solid transparent; */
            border-left: 10px solid transparent;
            border-right: 10px solid transparent;
        }

        .triangle:hover {
            /* 清除上边框 */
            /* border-top: none; */
            border-top: 0;
            border-bottom: 10px solid red;
            /* 改变指针 */
            cursor: pointer;
        }
    </style>
</head>

<body>
    <div class="box">
        内容
    </div>
    <a href="">sassa</a>

    <!-- 边框三角 -->
    <div class="triangle"></div>
</body>

</html>